1. Field of the Invention
This invention relates to a method and apparatus for molding plastic material, and, more particularly, this invention relates to compression molding of thermoplastic material.
2.State of the Art
In the well known art of injection molding, it is common practice to fill a mold quickly by injecting material through a gate orifice to fill the cavity and, then, holding pressure on the material source to make up for volume loss in the molded part as it cools. This is known as "packing" the cavity and results in inherently more precise moldings than processes which lack "packing" capability. Next the gate orifice is closed, either mechanically by means of a valve pin, or by freezing a small slug of the molding material in the orifice. Finally, the molded part is removed from the cavity. There exists a wide array of apparatus for gating and valving molten material into cavities under pressure and for maintaining molten material in distributors and manifolds during the "packing" phase of the molding cycle. Many are designed to insure minimum gate "vestige" marks on the parts as they are removed from the mold.
Compression molding of thermoplastics has some advantage over injection molding in certain applications where material properties may be degraded by relatively long residence times in the molten state or mechanical shearing as the material passes through small gate orifice under very high pressure. Compression molding is also capable of creating greater cavity pressure for forming fine details in very stiff or viscous materials which cause a large pressure drop when injected through a small orifice. There is a requirement for providing a precise charge of molten material to the cavity prior to closing the mold to form a part. Much of the technology of compression molding has been focused on improving the accuracy of metering these charges.
The problem of precise metering is addressed in patents covering methods and apparatus for improved metering means. However, none of the known metering means completely solves the problem of variable part weight due to variance in metering. Therefore, a significant problem with compression molded or hot forged thermoplastic parts remains that parts must be:
A. either under molded or over molded (flashed) in compression molds with fixed molding cavity volume, or
B. vary in some physical dimension in compression molds with variable (telescoping) molding cavity volume.
In practice, if no molded part dimensions are allowed to vary, molds are designed to deliberately flash excess material which necessitates a subsequent flash removal step in manufacturing. A further problem with present practices arises when the material being molded has a significant shrinkage factor associated with cooling. That occurs when a fixed displacent mold is initially closed on an excess charge of hot material and flashes, there is no way to make up for the volume loss as the part shrinks during cooling. The result is an under molded part which must still be trimmed of flash.